Lubricants are widely used to reduce the friction between the surfaces of moving parts and thereby reduce the wear and prevent damage to the parts. Most lubricants are comprised principally of a base stock which is generally a relatively high molecular weight hydrocarbon. In applications where the amount of pressure applied to the moving part is quite high, lubricants which are comprised of only hydrocarbon base stock tend to fail and the contacting parts are damaged. It is well known to add materials (lubricant additives) to increase the high pressure performance of lubricants. These materials are generally called "extreme pressure additives." Examples of extreme pressure additives are sulfurized materials such as sulfurized sperm whale oil or sulfurized jojoba oil.
Wax esters can be sulfurized from natural wax esters, such as from jojoba oil, and from synthesized wax esters, such as from vegetable oil triglycerides and liberated fatty acids. Natural wax esters have produced formulated sulfurized products that provide effective lubricant additive compositions. However, the problems with the natural wax esters are availability and cost. The vegetable oils have to be transesterified from the free fatty acids to produce the wax esters. For practical applications, however, the vegetable oil triglycerides have the disadvantage that cost-adding conversion steps are necessary to form a wax ester for sulfurization, rather than a direct sulfurization of the triglyceride. These extra processing steps make vegetable oil wax esters relatively uneconomical as lubricant additives.
Princen et al., "Development of New Crops for Industrial Raw Materials," J. Amer. Oil Chemists Soc., 61: 281-89, 1984, tried to sulfurize meadowfoam oil by sulfurizing the wax esters of the meadowfoam fatty acids. Princen et al. found that sulfurization of the unaltered triglyceride oil can react with sulfur under traditional sulfurization conditions to yield factice, a solid chemical rubber and an unacceptable lubricant additive. Factice forms due to cross-linking of hydrocarbon chains by polysulfide bonds to create an insoluble material. The first sulfurized wax ester of meadowfoam oil preparation in Princen et al. had good lubrication properties, but it corroded copper, foamed excessively, and thickened excessively under in-use conditions in a hot gear box. The second sulfurized wax ester of meadowfoam oil preparation in Princen et al. was made with different synthesis conditions, but the second preparation decreased copper corrosion and maintained lubricating properties but produced more foam and did not meet a thermal stability test. The third preparation in Princen et al. used wax esters from enriched monoene and diene acids. The diene wax ester (made from monoenoic fatty acids) resulted in increased wear scar but favorable other properties.
Wakim, U.S. Pat. No. 3,986,966, after demonstrating that sulfurization of triglycerides alone gave resinous products mostly insoluble in base oils, added nonwax methyl esters of a fatty acid of 18 to 22 carbon atoms which are monounsaturated to improve the solubility of the sulfurized triglycerides.
Hutchinson et al., U.S. Pat. No. 3,740,333 refers to sulfurized blends of lard oil and a saturated wax ester, the essential element of the wax ester being derived from a C.sub.10-16 saturated alcohol and a C.sub.18-22 unsaturated acid.
Kammann et al., "Sulfurized Vegetable Oil Products as Lubricant Additives," J. Amer. Oil Chemists Soc., 62:917-23, 1985, found that sulfurized vegetable triglyceride oils resulted in rubbery products, in some cases even at a 12% sulfur content. Kammann et al. tried using sulfurization with large amounts of methyl lardate, but obtained factice or near factice with meadowfoam oil. Kammann et al. was able to sulfurize only by adding 10% unsaturated oleic acid before sulfurization; however, the resulting product properties were undesirable as a lubricant additive.
There are many problems using a sulfurized vegetable oil and extra expenses making a wax ester from a triglyceride vegetable oil. Therefore, there exists a need in the art to find lubricant additive compositions that can use the relatively abundant supplies of vegetable oils without resorting to the multiple processing steps of making wax esters of the fatty acids. There further exists a need in the art to be able to sulfurize triglyceride vegetable oils for use as lubricant additives.